The present invention generally relates to semiconductor devices, and in particular to a quantum interference semiconductor device having a quantum point contact or restricted quantizing region.
With the progress of so-called submicron patterning, semiconductor devices having the structural elements of a size in the order of 1 micron or less are becoming available. In such devices, the wave nature of electrons and holes becomes conspicuous. In other words, one has to treat these particles as waves rather than classical particles. This tendency is enhanced with the decreasing device size.
The quantum interference semiconductor device is a semiconductor device that utilizes the wave nature of electrons and holes for operation. For example, the U.S. patent application Ser. No. 473,167 in which the assignee is the same assignee of the present invention, describes a quantum interference semiconductor device wherein an electron wave of a single electron is slitted into two or more electron waves and passed through respective quantum point contacts.
The quantum point contact is a region having a lateral size smaller than the de Broglie length of electrons in the direction perpendicular to the propagating direction and causes a quantization of the electron wave passing therethrough. Further, the quantum point contact is characterized by a restricted longitudinal length in the propagating direction of the electron wave such that the longitudinal length is smaller than the elastic or inelastic length of the electrons.
Upon passage of the quantum point contact, the electron wave experiences a change in the wavelength due to the quantized energy level or quantum level of the quantum point contact. Thereby, the phase of the electron wave is modulated. Upon merging of the electron waves which have passed through respective quantum point contacts, there occurs a constructive or destructive interference of the electron waves. Thus, when there occurs a constructive interference, a high level output is obtained at the output of the quantum interference semiconductor device, while when there occurs a destructive interference, a low level output is obtained.
FIGS. 1(A)-1(D) are diagrams for explaining the foregoing principle of the quantum interference semiconductor device having the quantum point contact, wherein FIG. 1(A) shows the device in the unbiased state in the plan view and FIG. 1(B) shows the device in the same state in the elevational cross section.
Referring to FIGS. 1(A) and 1(B), the quantum interference semiconductor device has an undoped channel layer 63 of GaAs and the like, and a doped layer 61 of n.sup.+ -type AlGaAs and the like, is provided on the layer 63 for supplying electrons to the channel layer 63. Between the channel layer 63 and the doped layer 61, an undoped AlGaAs layer 62 may be interposed. In such a structure, a two-dimensional electron gas 69 is formed in the channel layer 63 along the upper boundary thereof with the AlGaAs layer 62, as is well known.
On the upper major surface of the doped AlGaAs layer 61, there are provided a source electrode 65 and a drain electrode 66 in ohmic contact to with the underlying layer 61 as shown in the plan view of FIG. 1(A), and a pair of gate electrodes 67 and 68 are provided also on the upper major surface of the doped layer 61, in Schottky contact thereto therewith and located in correspondence to a region thereof between the source electrode 65 and the drain electrode 66.
FIGS. 1(C) and 1(D) show the same device in the biased state, wherein FIG. 1(C) corresponds to the plan view of FIG. 1(A) and FIG. 1(D) corresponds to the elevational cross section of FIG. 1(B).
Referring to FIGS. 1(C) and 1(D), a bias voltage is applied to the gate electrodes 67 and 68, and in correspondence thereto, there develop depletion regions 71 and 72 under the gate electrodes 67 and 68, respectively. As shown in the cross section of FIG. 1(D), the depletion regions 71 and 72 extend into the channel layer 63 and expel the electrons forming the two-dimensional electron gas 69. Thereby, the electrons are laterally confined into a region 74. The region 74 is confined also in the traveling direction of the electron waves such that the length of the region 74, measured in the traveling direction of the electron wave, is approximately smaller than the elastic and inelastic wavelength of electrons. Thus, region 74 forms a spot-like region called a quantum point contact.
As the separation between the gate electrodes 67 and 68 is set in the order of a few microns or less, one can confine the lateral width of this quantum point contact 74 approximately within the de Broglie length of electron by suitably choosing the bias voltage, and the energy level of electrons is quantized into one or more discrete quantum levels. It should be noted that the quantum level is changed in response to the extension of the depletion regions 71 and 72. In such a structure, the two-dimensional electron gas formed in the region between the source electrode 65 and the gate electrodes 67, 68 is connected to the two-dimensional electron gas formed in the region between the drain electrode 66 and the gate electrodes 67, 68, at the quantum point contact 74.
Thus, when an electron wave having an energy E.sub.F and a wavelength k.sub.F of: EQU k.sub.F =(2mE.sub.F).sup.1/2 /h
in the traveling direction has entered the quantum point contact 74, the wave number of the electron wave is changed to: EQU k.sub.F1 =[2m(E.sub.F -E.sub.01)].sup.1/2 /h
where E.sub.F represents the Fermi level, m represents the mass of an electron, h represents the Plank's constant, and E.sub.01 represents the quantum level formed in the region 74. In other words, the wavelength of the electron wave is controlled in response to the quantum level E.sub.01 of the quantum point contact 74.
FIGS. 2(A)-2(C) are diagrams showing a prior art quantum interference semiconductor device proposed previously in the foregoing U.S. patent application Ser. No. 473,167.
Referring to FIGS. 2(A) and 2(B), the quantum interference semiconductor device has a structure substantially identical with the structure of FIGS. 1(A)-1(D) except that there is provided another Schottky electrode 75 between the gate electrodes 67 and 68. Thereby, the Schottky electrode 75 forms a depletion region 75a in the channel layer 63 as shown in FIG. 2(B). In correspondence to this, there are formed two quantum point contacts 74a and 74b such that the quantum point contact 74a is located between the gate electrode 67 and the electrode 75 in the plan view and such that the quantum point contact 74b is located between the gate electrode 68 and the electrode 75.
In each of the quantum point contacts, the wavelength of the electron waves is modified according to the foregoing equation. Thereby, by adjusting the quantum level of the quantum point contacts 74a and 74b appropriately, one can modify the wavelength of the electron waves passing through the quantum point contacts separately.
FIG. 2(C) shows the principle of operation of the quantum interference semiconductor device of FIGS. 2(A) and 2(B).
Referring to FIG. 2(C), an electron wave W0 of a single electron emitted from the source electrode 65 is split into two electron waves W1 and W2 which pass through the respective quantum point contacts 74a and 74b. Upon passing through the quantum point contacts 74a and 74b, the electron waves 74a and 74b merge with each other and undergo an interference therebetween. It should be noted that the absolute value of the wave function of an electron wave represents the probability of finding an electron and thus, there occurs conduction of the device when the electron waves W1 and W2 cause a constructive interference. On the other hand, when these electron waves cause a destructive interference, the semiconductor device is in the turned off state. Such an interference of the electron waves is controlled by controlling the quantum level E.sub.01 of the quantum point contacts 74a and 74b independently, which in turn can be controlled by controlling the respective lateral extensions of the depletion regions 71 and 72 under the gate electrodes 67 and 68 independently in response to the corresponding gate voltages applied to the gate electrodes 67 and 68.
In this conventional quantum interference device, however, there arises a problem in that, as the electrode 75 between the gate electrodes 67 and 68 is in the floating state, the electric potential of the electrode 75 is not fixed but changes, variously, in response to the voltage between the gate electrodes 67 and 68 or in response to the voltage between the source and drain electrodes 65 and 66. It should be noted that when there is a potential gradient along the surface of the layer 61 between the electrode 67 and the electrode 68, the voltage level appearing on the electrode 75 assumes a value intermediate (i.e., between) the voltage level of the electrode 67 and the voltage level of the electrode 68.
It should be noted that the extension of the depletion region 75a , formed under the electrode 75 is determined in accordance with, and response to, the intermediate potential level of the electrode 75. Thus, when the potential level of the electrode 75 is changed, such a change induces a change in the quantum level of the quantum point contacts 74a and 74b by modifying the extension of the depletion region 75a under the electrode 75. Thus, such an uncontrolled change of the potential level of the electrode 75 can induce unstable device operation. Further, the change of the potential level of the electrode 75 may be caused by other reasons such as irradiation of light.
It should be noted that a similar problem occurs also when the depletion region 75a is formed by forming a depression or inactive region in the layer 61 in place of the electrode 75.